In the fields of medicine and agriculture it is often desirable to maintain an effective concentration of an active agent, for example a pesticide, herbicide, fertilizer or drug, at some site of action for a prolonged time. One method of achieving this goal is to deliver a large excess of the agent so that even though it is metabolized, excreted, or degraded, sufficient remains to maintain the effective dose. This approach is not only wasteful of the active agent but maintaining such a large excess during the early portion of the delivery period often leads to overdose-related side effects. A better pattern of delivery is to dispense the agent from a sustained release delivery system which releases the active agent at a slow rate throughout the delivery period. Many of these devices have been used and are described elsewhere, for example in "Controlled Release of Biologically Active Agents," A. C. Tanquary and R. E. Lacey (Eds) Plenum Press, New York, 1974 and the references therein. A plurality of the dispensers encompassed by this invention can also be used to deliver active agents at a more or less constant rate throughout the delivery period but in addition they can be programed to deliver the agents in periodic pulses at fixed time intervals or some other programed non-constant delivery regime. Many situations exist where periodic or non-constant delivery is desirable.